Acoustic wave sensors use a detection arrangement that is based on perturbations to mechanical or acoustic waves. As an acoustic wave propagates through or on the surface of the acoustive wave sensor material, any changes to the physical or chemical characteristics of the wave path may affect the velocity and/or amplitude of the acoustic wave. These changes may be correlated to the corresponding physical, chemical, or biological quantities being measured to provide sensing.
Recreational inland lakes, public beaches, and parks, which may be designated for recreational purposes including fishing, swimming, etc., may be closed if there are sufficiently high levels of bacterial contamination, or other sufficiently high levels of chemical, heavy metal, and biological contaminants. At present, continuous monitoring of both bacterial and chemical contaminants may not be available. A source of certain environmental problems, which may lead to restrictions of watersheds for human use, may result from sufficiently high levels of Coliform bacteria, in particular, Escherichia Coli. Such levels of this bacteria may be detected by routine sampling or when a public health incident indicates there may be such biological contamination. It may take about 2 days to culture the water samples and determine the basis for any such quality problems. It is believed the detection of chemical and bacteria contamination of drinking and recreation waters would be useful.
There may be various biological and chemical sensors, using fiber optics, chemical interactions, and various fluorescence approaches. Such sensors may, however, have various weaknesses, such as, for example, low sensitivity, selectivity, or an inability to be hybridized or integrated into sensing chip technology. Acoustic wave (AW) sensors, however, may be better suited for use in biological and chemical detection. As discussed in D. S. Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, G. C. Frye, H. Wohltjen, “Acoustic Wave Sensor—Theory, Design, and Physico-Chemical Applications”, Academic Press, (1997), acoustic wave sensors may use piezoelectric crystals, which may allow transduction between electrical and acoustic energies. The AW sensor may use piezoelectric material to convert a high frequency signal into an acoustic wave, and the higher frequency may enable the sensor to be more sensitive to surface perturbations.
Piezoelectric materials used for acoustic wave sensors may include quartz (SiO2), lithium niobate (LiNbO3), zinc oxide (ZnO), and others. Each of these materials may possess specific advantages and disadvantages, which may relate to, for example, cost, temperature dependence, attenuation, and propagation velocity. Such materials may, however, have limited transverse acoustic wave velocities, low electromechanical coupling coefficients, non-linear temperature coefficients, and may react chemically with the environment. (See the background information in C. Caliendo, G. Saggio, P. Veradi, E. Verona, “Piezoelectric AlN Film for SAW Device Applications”, Proc. IEEE Ultrasonic Symp., 249-252, (1992) and K. Kaya, Y. Kanno, I. Takahashi, Y. Shibata, T. Hirai, “Synthesis of AlN Thin Films on Sapphire Substrates by Chemical Vapor Deposition of AlCl3—NH3 Systems and Surface Acoustic Wave Properties”, Jpn. J. Appl. Phys. Vol. 35, 2782-2787, (1996) and G. Carlotti et al., “The Elastic Constants of Sputtered AlN Films”, Proc. IEEE Ultrasonic Symp., 353, (1992)).
A surface acoustic wave (SAW) sensor may have further disadvantages in liquid if there is sensitivity to viscous damping. In such a case, surface transverse wave (STW) devices may be more suitable in liquid environments. Unlike SAW devices, STW devices use a “shear-horizontal” or “lamb” wave that is horizontally polarized as it propagates across a surface. Accordingly, STW sensors may operate in liquid while maintaining a high mass sensitivity without severe attenuation. (See R. L. Baer, C. A. Flory, M. Tom-Moy, D. S. Solomon, “STW Chemical Sensors”, Proc. IEEE Ultrasonic Symp. 293-298 (1992)). There may be other acoustic mode devices that may be suitable for liquid environments, including Shear Horizontal Acoustic Plate Mode (SO-APM) arrangements and Flexural Plate Wave arrangements. Such arrangements may, however, suffer from low mass sensitivity or structure fragility. It is therefore believed that STW arrangements may be better adapted for use in biological and chemical detection.